Verticillium Wilt
of Trees and Shrubs
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Verticillium wilt,
caused by the fungi Verticillium albo-atrum and V. dahliae, is a serious
vascular disease of hundreds of woody and herbaceous plant hosts. Food
crop hosts include everything from raspberries and strawberries, to
tomatoes and potatoes.
Some of the many common woody ornamental host plants include
ash, barberry, catalpa, elm, magnolia, maple, Russian olive, redbud,
smoketree, tuliptree, and viburnum. One group of plants not susceptible
to Verticillium wilt are all the gymnosperms, including conifers such as
pine and spruce. While many landscape plants are affected, Verticillium
wilt is not a major problem in natural forested areas
Diagnostic Symptoms |
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Wilting of leaves and dieback of branches, often one at a time or on one side of
the tree, are the most severe symptoms. This can occur over a number of years,
with remission of symptoms in some years, or can rapidly progress to plant death
in a year or two (Figure 1). Other symptoms of Verticillium wilt may include:
marginal browning and scorch of leaves, abnormally large seed crops, small
leaves, stunting, poor annual growth, and sparse foliage.
Sometimes large areas of cambial tissue die from infections by the fungus and
opportunistic fungi such as Nectria develop in elongated cankers. Late season
infections may not be noticeable until plants come out of dormancy with branch
dieback evident.
All of the above symptoms can also be caused by other stress factors. A good
field symptom that can set Verticillium wilt apart diagnostically is the
discoloration of xylem and cambial tissue, visible as streaks if you cut into
the wood. This discoloration is variable for different plants: generally
greenish to blackish on maple, yellowish green on smoketree, and brown on ash.
This streaking is not totally diagnostic on two counts: 1) other fungi and other
factors can cause discoloration; and 2) on some hosts and on youngest twigs,
infection is not always accompanied by discoloration. However, vascular
discoloration is a good field symptom that can then be followed up for
confirmation by a diagnostic lab such as The Ohio State University Plant & Pest
Diagnostic Clinic.Disease
Cycle and Conditions Favoring Disease
The Verticillium fungus can survive for many years in soil, making effective
crop rotation difficult. The fungus infects plant roots through wounds and in
some cases direct penetration of root tissue. Verticillium also is transmitted
from plant to plant by grafting and budding.
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From root infections, the fungus spreads upward in the plant through the
vascular stream. The results of infection are tissue damage and plugging of
xylem, robbing stems and leaves of needed water and minerals. The fungus is
returned to the soil as plant parts fall or die, and tiny resistant fungal
microsclerotia are spread by wind, in soil and on equipment. Many weed hosts are
also susceptible; therefore the cycle of contaminated soil is hard to break.
Development of Verticillium wilt is favored by factors that stress roots,
including wounding and droughty conditions.
Control
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1. Disease
resistance. If Verticillium wilt is diagnosed at a
particular landscape or
nursery site it is prudent to
replant into that area with a plant that
exhibits resistance to
this disease. A few common examples of plants
typically free of
this disease include: crabapple, mountain ash, beech,
birch,
boxwood, dogwood, sweet gum, hawthorn, holly, katsuratree,
honeylocust, oak, pear, London planetree and sycamore,
rhododendron,
willow, and zelkova. The red maple cultivars
Armstrong, Autumn Flame, Bowhall, October Glory, Red
Sunset, Scarlet and Schlessinger have also
been reported as
resistant. |
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2. Keep plants
as healthy as possible. Proper transplanting practices, proper water
management
to avoid droughts, a good fertility program, and pruning out
dead branches are all good plant
health care management practices. These
can help limit infections and help limit the effects of
these infections
to some extent. Pruning out infected branches is useful as a general
horticultural practice for overall plant vigor and aesthetics, but does
not eliminate
Verticillium from the plant since infections spread from the
roots.
Fungicides are not effective for control of this disease.
Ohio State University
Extension Fact Sheet, Plant Pathology,2021 Coffey Road, Columbus, OH
43210-1087
Jim Chatfield, Stephen Nameth, Nancy Taylor. HYG-3053-96
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